The temperature fields at a coupled boundary patch do not match the temperature field of the adjacent coupled boundary patch of another mesh region. Both patches are defined as "mappedWall" with the "compressible::turbulentTemperatureCoupledBaffleMixed" boundary condition.

The coupling in chtMultiRegion*Foam is explicit, i.e. it uses uptodate coefficients on either side when evaluating the coupled boundary condition. Only upon convergence will the temperatures/fluxes be equal.

No, the whole system should be converged. Heat transfer is affected by local properties. E.g. heat transfer coefficient depends on turbulence which depends on the flow. Note that the multiRegionHeater tutorials are simplified cases and might not even be steady.

The reason for the unequal distribution was due to some odd behaviour of the reader plugin in paraview. The actual temperature fields were correct, but not correct displayed. Using another version of paraview, all CHT temperature fields match perfectly.